The present invention concerns a multiple wavelength source, in particular for optical communication using a plurality of wavelength multiplexed channels, comprising a broadband light source coupled to discrete wavelength separator means.
There is a trend in optical communication systems to use wavelength multiplexing which enables a plurality of communication channels to be conveyed on a common optical transmission medium. Each channel consists of a source of a particular wavelength that is modulated by the information to be transmitted on that channel, coupled to others on a common medium, usually an optical fiber, and then separated from the other channels and demodulated to reconstitute the transmitted information.
Transmitting a plurality of multiplexed channels requires the carrier wavelengths of the various channels to be precisely defined in the spectrum and to have a high spectral purity.
Initially, the technology offered only independent sources, in particular laser diodes, to generate the wavelengths of the various channels. Apart from the high initial cost of a multiplicity of sources, this solution is also unsatisfactory in that independent sources respond independently of each other to their conditions of use, leading to high additional costs for initially setting and subsequently maintaining the adjustment of each source.
Consideration has therefore been given to obtaining the wavelengths of the various channels from a single source, in order to obtain at least uniform behavior of a source of multiple wavelengths, so that the adjustment mentioned can be simplified.
The principle of a solution of this kind is described in the article xe2x80x9cSpectrum-Sliced Fiber Amplifier Light Source for Multichannel WDM Applicationsxe2x80x9d by J. S. Lee and Y. C. Chung published in IEEE Photonics Technology Letters, Vol. 5, No. 12, December 1993.
This solution employs a broadband light source coupled to discrete wavelength separator means. After mentioning light-emitting diode and superluminescent light sources, the article recommends the use of a spontaneous emission amplifier consisting of an erbium doped optical fiber pumped at 1.48 xcexcm in series with an isolator to prevent any laser effect.
The wavelength separator means are not described, but merely shown as a wavelength demultiplexer, i.e., broadly speaking, as a set of filters driven in parallel and supplying selected wavelengths at individual outputs. This provides only limited selectivity and spectral purity and introduces high losses for each of the selected wavelengths.
The system, not yet implemented experimentally, could provide about 20 channels separated by 0.6 nm.
The present invention is aimed at improving a solution of this kind to provide a greater number of channels at substantially reduced cost, providing selected discrete wavelengths at a significantly higher power level.
In accordance with the invention, the multiple wavelength source defined at the beginning of this text is characterized in that said wavelength separator means comprise cascaded refractive Bragg gratings each reflecting one of said discrete wavelengths to a source output.
Said additional means preferably comprise an absorbent termination after the last of the refractive Bragg gratings.
The set of refractive Bragg gratings therefore reflects the set of discrete wavelengths selectively. A signal made up of only the wanted wavelengths is thus obtained directly. Each wavelength is narrowly selected and therefore has high spectral purity. Because of the absence of intermediate wavelengths between the wavelengths selected in this way, their subsequent spatial separation is facilitated.
In one embodiment of the invention said refractive gratings are disposed at the output of said light source and select said discrete wavelengths in the broadband spectrum that the latter produces.
A directional coupler routes the broadband emission towards the cascade of Bragg gratings and the absorbent termination that terminates it and routes the reflected wavelengths towards said source output.
The coupler is advantageously located at the output of said light source.
An optical isolator is advantageously disposed between said source and said directional coupler.
Said light source is preferably an erbium-doped fluoride-based optical fiber.
The end of said light source opposite its output is advantageously terminated by a mirror.
In another embodiment, the end of said light source opposite its output is coupled to said cascade of Bragg gratings terminated by said absorption means.
Spontaneous emission amplified by passing twice through the fiber is thus favored at the wavelengths reflected by said cascaded Bragg gratings.
The various objects and features of the present invention will emerge more clearly from the following description of one embodiment of the invention given by way of non-limiting example with reference to the appended figures.